1. Technical Field of the Invention
The present invention relates to a method and apparatus for supplying, to a radiation detector, a bias voltage for driving the radiation detector, and in particular to a method and apparatus for supplying the bias voltage to a semiconductor detector which detects radiation, such as X-rays, in a state where a voltage is fed to the detector.
2. Related Art
In recent years, in the field of medical diagnostic devices, including X-ray CT scanners and dental X-ray panoramic apparatuses, digital radiation detectors have been adopted by many devices. In such detectors, particular attention is paid to a photon counting type of detector. Such examples are known by JPA 2000-131440 and JPA 2009-268892.
This X-ray detector has a layer member composed of a compound semiconductor which directly converts an incident X-ray beam to an electrical signal which has an amount of electric charges corresponding to an intensity of the X-ray beam, and a pair of charging and collecting electrodes each disposed on both surfaces of the layer member. The compound semiconductor composing the layer member is for example CdTe (cadmium telluride) or CdZnTe (cadmium zinc telluride).
The collecting electrode is sectioned, pixel by pixel, into a group of pixels one- or two-dimensionally arrayed. For the photon counting method, an X-ray beam is detected and its output signal is processed, on the assumption that the X-ray beam is a stream of particles. That is, electrical signals collected by plural collecting electrodes, which are assigned to plural pixels, are processed into digital signals depending on X-ray energy and the number of X-ray photons.
Some circuits for the processing are assembled, as an ASIC (application specific integration circuit) layer, into a layer which contacts the collecting electrode. As a matter of course, the electric signals detected in the X-ray detector can be processed by an electric circuit separated from the detector, not limited to the case where the signals are processed by the ASIC layer. In addition, the detector may adopt an integral-method signal acquisition technique, not limited to adopting the photon counting method.
However, it is difficult to homogenously produce all of compound semiconductors which compose the pixels of an X-ray detector. When being used, the detector receives a certain bias voltage at the collecting electrodes, resulting in flow of a dark current and/or occurrence of a polarization effect.
The dark current is a minute current flowing from the charging electrode to collecting electrodes due to the applied bias voltage, even when there are no incident X-ray beams. The dark current is composed of a current component flowing along edge surfaces of the compound semiconductor, i.e., edge surfaces of the X-ray detector, depending on the surface resistance, and a current component flowing through the compound semiconductor, i.e., through respective pixels. The former current component depends on circumstantial factors such as a surface state and a surface resistance of the compound semiconductor, temperature, and humidity. The latter current component changes due to irregularities in crystal structures of a compound semiconductor when being manufactured, how substrate portions are bonded, how bump bonding is, temperature, humidity, and temperature when the bump bonding is performed
The polarization effect is a phenomenon which will cause a detector output to drift gradually during a continuing application of the bias voltage. This phenomenon will occur particularly in compound semiconductors with Schottky structures.
When the dark current flows and/or the polarization effect is caused, the detection performance of each pixel becomes unstable, causing irregularities in output signals from the respective pixels among the pixels, even when there is an incidence of the same amount of X-ray flux at each pixel. In a case where the irregularities in the output signals become excessively large to be over its allowance range, such pixels should be treated as defective pixels. It is noted that, if the polarization effect is excessive, output signals from pixels may change so as to exceed a tolerance range during performing a scan. In this case, the defective pixels themselves increase in number during the scan.
In addition, if there is occurrence of dark current and/or the polarization effect, a dynamic range during which strength of X-rays is detectable correctly narrows. Hence, if such events occur, the number of bad pixels, such as defective pixels with no output, pixels whose outputs are unstable, and/or pixels indicating abnormal outputs compared with other pixels, will become larger, even for incidence of X-rays with higher amounts of flux.
Conventionally it is known to arrange a guard ring around the pixels to physically raise electric resistance to prevent a component of the dark current transmitted along edges of the detector. This guard ring provides the detector sides with a barrier effect against such dark current components. This guard ring is, to some extent, helpful for preventing the dark current from flowing along the sides.
In this way, even if the same X-ray strength arrives at each pixel, larger influences caused by irregularities in the dark current and/or the polarization effect will cause each pixel to output inconstant signals. In order to prevent differences of detection characteristics which are due to the irregularities in the output signals, it is currently necessary to previously check operational conditions of an X-ray detector, which operational conditions give the detector a stable detection performance. Practically it is necessary to take various countermeasures such as regular reset of the bias voltage and control of temperature and humidity in the environment.
However, when such countermeasures are performed, it is necessary to previously test manufactured X-ray detectors to select ones which have a sufficient detection performance for use. A yield ratio in this selection depends largely on production lots. In particular, compound semiconductors such as CdTe have larger irregularities in their production. Hence, keeping such irregularities within a given range results in a rise in production cost of X-ray detectors.
Nevertheless, in addition to the foregoing countermeasures, it is needed to employ post processes for output signals from an X-ray detector, where the post processes include a process for defective pixels and various correction processes for uniformizing an image.
Therefore, In the X-ray detectors which use the compound semiconductor, use conditions are largely restricted due to irregularities in the dark current and/or the polarization effect, and an amount of calculation necessary for the post processes increases.
The foregoing drawbacks are always associated with occurrence of the dark current and/or the polarization effect and not related to types of X-ray detectors. Even an integration type of detector, which outputs, as an X-ray detection signal, an integrated value (for a given period of time) of an output signal from the detector is confronted with the same drawbacks. Further, gamma rays have the same situation as the above, not limited to the X-rays.